Including page fragments

I have lots of page fragments and I want to include only one of them on bsp based on a value of a variable.

How can I do this?

I wrote <%@ include file="pagefragment.htm"%> into a string and then put that string on the layout, but surprise, surprise, it doesn't work.

Can I somehow use

<%@ include file="pagefragment.htm"%>

and define the relative URL dynamically?

Any other ways to include a page fragment?